Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation. In the process of recovering hydrocarbons from subterranean formations, it is common practice to treat a hydrocarbon-bearing formation with a pressurized fluid to provide flow channels, i.e., to fracture the formation, or to use such fluids to transport and place proppant to facilitate flow of the hydrocarbons to the wellbore.
Well treatment fluids, particularly those used in fracturing, may comprise a water or oil based fluid incorporating a thickening agent, normally a polymeric material. Typical polymeric thickening agents for use in such fluids comprise galactomannan gums, such as guar and substituted guars such as hydroxypropyl guar and carboxymethylhydroxypropyl guar (CMHPG). Cellulosic polymers such as carboxymethyl cellulose may also be used, as well as synthetic polymers such as polyacrylamide. To increase the viscosity, and, therefore, the proppant carrying ability of the fracturing fluid, as well as increase its high temperature stability, the polymeric materials may be crosslinked. Crosslinking a polymer solution may increase the steady shear viscosity up to two orders of magnitude. For well stimulation treatments, particularly hydraulic fracturing, this may creating fracture width and transporting and place proppant in the fracture.
By necessity, well treatment fluids are prepared on the surface, and then pumped through tubing in the wellbore to the hydrocarbon-bearing subterranean formation. While high viscosity, thickened fluid is highly desirable within the formation in order to transfer hydraulic pressure efficiently to the rock and to reduce fluid leak-off, large amounts of energy are consumed to pump such fluids through the tubing into the formation. To reduce the amount of energy consumed, various methods of delaying crosslinking have been developed. For example, metal complexing ligands, sometimes termed a chelant, may be employed to initially bind the metal, but on a controlled basis, and subsequently exchanged for coordination of the polymer by the metal. When two separate polymer molecules, or strands, are complexed by the metal, viscosity builds through the three-dimensional network. Other crosslinking delay methods which have been employed include physical sequestration of the crosslinker from the polymer by means such as encapsulating, or coating, the crosslinker, or delaying its contact into the polymer-bearing phase by emulsifying it. Other methods include dissolution controlled release of slowly soluble, crosslinker-containing solids, and delayed alteration of the pH of the combined polymer/crosslinker solution. These techniques allow the pumping of a relatively less viscous fluid having relatively low friction pressures within the well tubing with crosslinking being effected near or in the formation so that the properties of thickened crosslinked fluid are available at the rock face.
During the process of obtaining hydrocarbons (including the acts described above), undesirable materials, such as water, may also travel through the formation in the vicinity of the wellbore and ultimately enter the wellbore. The presence of water may be an issue in numerous formations, such as, for example, sand, sandstone, chalk, limestone, etc. The rate at which the water appears in the wellbore may be slowed through the use of various technologies directed to preventing undesirable materials from entering the wellbore. Conventional water shut off techniques range from mechanical to chemical treatment strategies.